Recently, research and development of WDM-PONs is vigorously conducted so as to increase the capacity of networks and improve the transmission rate. In particular, WDM-PONs are regarded as best suited to accommodate video/image-based services. In order to implement WDM-PONs economically, it has recently been proposed to use an F-P LD (Fabry-Perot Laser Diode), which is wavelength-locked by the injected incoherent ASE (Amplified Spontaneous Emission) light from outside, as a light source of WDM-PONs. However, when a fiber fails unexpectedly in any type of WDM-PON system, the failure must be quickly grasped so as to restore the system, or the reliability of services provided to ONTs (Optical Network Terminations) is seriously degraded.
In order to monitor failure positions on fibers, an OTDR (Optical Time Domain Reflectometry) is generally used. This device is operated in the following manner: it injects pulse-type monitoring light, which has a short time width, into fibers and receives light backscattered by fine impurities inside the fibers. The OTDR then receives light reflected by connection units, particularly optical devices including a coupler, a connector, an AWG (Arrayed Waveguide Grating), and a WDM, by using a photo detector having good sensitivity. The intensity of received light is given as a function of distance. When the injected monitoring light propagates through the fibers, a small amount of the light is backscattered, but it undergoes a large degree of reflection where a failure has occurred. Therefore, the time-based function of received backscattered light becomes discontinuous due to reflected waves at a specific point of time, and the distance (position) of the failure is known by associating the point of time with the speed of light inside the fibers.
However, there are a number of difficulties in applying the above-mentioned method for monitoring failure positions to a WDM-PON system. As known in the art, different wavelengths are used from a remote node to respective ONTs. If a specific distributed fiber has failed, different wavelengths of monitoring light needs to be used to monitor the failure positions. A wavelength-tunable light source may be used to this end, but it is economically unfavorable.
In an attempt to solve the problems occurring when a wavelength-tunable light source is used, it has been proposed to use a DFB LD (Distributed Feedback Laser Diode) as a conventional light source for signal transmission and, when a failure has occurred, identify a faulty channel by means of a receiver of a CO (Central Office), so that the faulty channel's DFB LD for signal transmission is used as a monitoring light source. However, this approach is not economical because expensive DFB LDs are used. Furthermore, a switch for toggling between transmission mode and monitoring mode is necessary for each channel. The resulting increase in channels renders the transmission unit complicated.